Analysis of Factors Affecting Toughness of EH36 Ship Plate Core
properties of EH36 ship plates designed and produced by two different compositions , and analyzes the factors affecting the impact toughness of EH36 ship plates .
EH36 ship plate that meets the performance requirements of the classification society can be obtained .
Key words thick gauge ship plate; core; toughness
With the development of container ships , coastal liquefied natural gas carriers and storage equipment , the types of ship boards required have changed from ordinary toughness grades such as A and B grades of ordinary strength ship boards and AH32 and AH36 high-strength ship boards to high-toughness ship boards Development involves grades such as D , DH32 , DH36 , E , EH32 , EH36 , etc. In order to reduce the weight of the hull, the thickness of the steel plate is gradually increasing . For this reason, the development of thick-gauge and high-toughness ship plates is on the agenda, among which high-strength EH36 is a brand with a large amount .
The normal delivery toughness index of EH36 ship plate is -40T V -notch impact energy (longitudinal) not less than 41J . When the product thickness is less than or equal to 40mm , the impact sample is a near-surface sample. When the product thickness is greater than 40mm , the sample axis Located at 1/4 thickness . However, in the certification process of the classification society, it is required that the notched impact energy (longitudinal) of the -60TV type is not less than 41J . There must be a guarantee . The existing public information does not analyze the measures to ensure the toughness of the thickness core. This paper analyzes the relevant influencing factors of the impact toughness of the thick gauge ship plate .
1 The performance of EH36 produced by two schemes
Scheme 1 is to use the stock AH36 billet, conduct a series of performance tests after controlled rolling, controlled cooling and normalizing. The results show that its strength is slightly lower than the standard requirements, and the low temperature toughness cannot meet the requirements .
Scheme 2 is to obtain the required low temperature toughness performance of EH36 after optimizing the composition design , billet purity, controlled rolling, controlled cooling and heat treatment process . The composition design of the two schemes is shown in Table 1 , the strength and plasticity are shown in Table 2 , and the impact toughness series temperature diagrams are shown in Figures 1 and 2 .
Table 3 lists the structure and inclusions of the steel plates of the two schemes .
Chemical composition design of two schemes
plan |
C |
mn |
Si |
P |
S |
ALs |
Nb + V + Ti + Ni |
As + Sn |
|
1 |
0 . 15 |
1 . 34 |
0 . 30 |
0 . 016 |
0 . 005 |
0 . 024 |
0 . 04 |
0 . 08 |
|
2 |
0 . 12 |
1 . 50 |
0 . 30 |
0 . 011 |
0 . 002 |
0 . 024 |
0 . 06 |
0 . 03 |
Table 2 Strength and Plasticity
plan
|
1 |
Horizontal 1 /4
Table 2 Strength and Plasticity |
350 |
500 |
30 |
qualified |
||||||||||||||||||||||||||||
|
Horizontal 1/2 thickness |
340 |
490 |
25 |
qualified |
|||||||||||||||||||||||||||||
|
2 |
Horizontal 1/4 thickness |
395 |
520 |
35.5 |
qualified |
||||||||||||||||||||||||||||
|
Horizontal 1/2 thickness |
390 |
520 |
31 |
qualified |
|||||||||||||||||||||||||||||
|
standard requirement |
|
355 |
490-630 * |
Nose 21 |
qualified |
|
1 |
Horizontal 1 /4
Table 2 Strength and Plasticity |
350 |
500 |
30 |
qualified |
||||||||||||||||||||||||||||
|
Horizontal 1/2 thickness |
340 |
490 |
25 |
qualified |
|||||||||||||||||||||||||||||
|
2 |
Horizontal 1/4 thickness |
395 |
520 |
35.5 |
qualified |
||||||||||||||||||||||||||||
|
Horizontal 1/2 thickness |
390 |
520 |
31 |
qualified |
|||||||||||||||||||||||||||||
|
standard requirement |
|
355 |
490-630 * |
Nose 21 |
qualified |
Table 2 Strength and Plasticity
plan
Sampling position
Yield strength Tensile strength Elongation Cold bending / MPa /MPa / %
plan |
organize |
Ribbon level |
Inclusion level |
grain size
/ level |
|
1 |
F + P , the grain size is uneven, the center is severely segregated, there is a small amount of bainite in the center, and the center is cracked |
4 |
A1.5C1.0e |
8 |
|
2 |
F + P , uniform grain |
1 |
C0.5 , D0.5 |
9 |
: All testing standards are GB / T10561-2005 .
2 Analysis of the reasons affecting the toughness of EH36 ship plate steel
2 . 1 Influence of chemical composition of steel
scheme 1 and scheme 2 includes carbon , manganese content , alloy elements niobium , vanadium , titanium and harmful elements phosphorus , sulfur , arsenic , tin, etc.
Carbon element forms interstitial solid-solution strengthening to the matrix in steel and improves the strength of the material, but its embrittlement vector is 0.37 t3 C/MPa [ ° . In order to ensure excellent low-temperature impact toughness, reducing carbon content is a better means, and reducing carbon is beneficial to the welding performance of hull steel . In order to make up for the loss of strength after carbon reduction, scheme 2 increases the manganese content by 0 . 16% , because manganese does not lose toughness while increasing strength, and its embrittlement vector is 0C/MPa [ 1 ] .
The role of microalloying elements in steel is very significant . Compared with scheme 1 , scheme 2 newly adds titanium. The precipitation of titanium inhibits the growth of austenite grains during the heating process. During the rolling process, through the strain-induced mechanism, the precipitated fine niobium , titanium carbide and The nitride pins on the grain boundaries and dislocations, inhibits the progress of austenite recrystallization and the growth of grains, and plays the role of refining grains . During the rolling and cooling process, the dispersed precipitation of carbon and nitride of niobium vanadium plays a role of precipitation strengthening .
Phosphorus and sulfur elements in steel are harmful to toughness. Scheme 2 reduces the phosphorus content in steel by double slag method, and the total content of phosphorus and sulfur is reduced by 0 . 008% .
The residual elements arsenic and tin in steel are harmful to toughness, because their electronegativity and size factors make them easy to segregate at grain boundaries and reduce the cohesion of grain boundaries . The impact on the macroscopic properties is that the fracture work of the steel is reduced, and the impact toughness is significantly reduced . Therefore, plan 2 reduces the content of arsenic and tin in steel, which is beneficial to improve toughness .
2 . 2 Reduce inclusions and inclusion denaturation in steel
The inclusions in Scheme 1 are Class A , sulfide 1-5 , with a length of 310^m and a width of 2^m , and Type C silicate 1.0e , with a length of 90^m and a width of 7^m . The large size of the inclusions affects the continuity of the steel, forms microcracks in the steel and promotes the expansion of the microcracks through the micropore aggregation mechanism, thus significantly reducing the plasticity and toughness of the steel material .
The purity of the steel is improved by vacuum treatment and the inclusions are spheroidized by calcium treatment . The type of inclusions in Scheme 2 is changed and the level is reduced. There is no sulfide inclusion, and the level of silicate inclusion is reduced to 0.5 in the fine category. The size is 8^m in length and 2^m in width . Improvement in inclusions fundamentally increases toughness .
2.3 Influence of low quality of billet
Tissue observation found that the center of the plan 1 steel plate was seriously segregated, there was a small amount of bainite in the center, the center was cracked, and the banded structure was serious, as shown in the figure
3. As shown in Figure 4 . For EH36 series steel grades, the conventional structure is iron
Fig.3 Banded tissue in the core of the steel plate in Scheme 1 ( 100 X )
Figure 4 Scheme 1 steel plate with cracks in the center and a small amount of bainite ( 500 X ) matrix plus pearlite. Bainite in the center is an abnormal structure. Low-magnification detection shows that the center of the billet is segregated to B1-5 grade, and the center shrinks loosely. Level 1.5 . Segregation in the center of the slab causes abnormal structure of the steel plate . By reducing the superheat in the continuous casting process, continuous casting light reduction and other measures, the low-fold quality of the slab in Scheme 2 was significantly improved, reaching C0.5 grade, and there was no abnormal structure in the center of the steel plate metallographic observation .
2.4 Effect of grain refinement and grain uniformity The structures obtained by the process design of the two schemes are both ferrite and pearlite. Under the same structure type, the grain size has a great influence on the toughness. Grain refinement increases the number of grain boundaries, which are obstacles to the movement of dislocations and increase the strength. The grain boundary can also limit the plastic deformation to a certain range, make the deformation uniform, and improve the plasticity of the material. The grain boundaries are in turn a resistance to crack propagation, thus improving the toughness of the material. Hall－ _ The Petch formula expresses the relationship between the grain size D and the ductile-brittle transition temperature Tc [ 2 ] :
Tc = A - mD - 1/2 ( 1 )
Where Tc is the ductile-brittle transition temperature;
D grain size;
A 、 m Constant, for structural steel m is 12 °C / mm 1/2 .
The average grain diameter of Scheme 2 is 15.9 pm compared with the average grain diameter of Scheme 1 of 22.5pm , which is 6.6pm thinner. According to the theoretical calculation of formula ( 1 ), the ductile-brittle transition temperature Tc of Scheme 2 is lower than that of Scheme 1 15C .
From the comparison of impact toughness in Figure 1 and Figure 2 , compared with Scheme 1 , the ductile-brittle transition temperature Tc of the actual scheme 2 is reduced by about 30C . In addition to the contribution of grain refinement and the improvement of billet purity, the grain uniformity is also important factors affecting resilience .
Scheme 2 , an optimized process design is used to obtain a fine and uniform structure . During the heating process, the heating temperature is controlled to ensure that the niobium and vanadium carbonitrides in the steel dissolve into the steel, thereby refining the grains in the subsequent rolling and cooling, while controlling the maximum heating temperature to prevent the coarsening of the original austenite grains . Control the rolling process: When rolling in the austenite recrystallization zone, the deformation should be fully penetrated to the central part in the thickness direction, so that the austenite recrystallization can be fully carried out, and the grain size can be uniformly refined; in the austenite When rolling in the non-recrystallized area, a certain deformation rate ( M50% ) is guaranteed, and at the same time, the deformation in the thickness direction of the steel plate is sufficient and uniform, and the cooling rate after rolling is controlled to obtain a refined as-rolled structure . During the normalizing process, the heating temperature is designed according to Ac3 + ( 50 -100 ) C. If the normalizing temperature is too high, the structure will be coarse. If the normalizing temperature is too low, the degree of uniformity of the structure will not be enough, and the best performance cannot be obtained . The normalizing process makes the abnormal structure produced in the rolling process, such as uneven grains in the thickness direction , banded structure and other structural defects, be improved by recrystallization and homogenized structure, and fine equiaxed ferrite plus uniformly distributed blocks can be obtained. pearlite structure, thereby improving its mechanical properties and process performance .
2.5 The influence of precipitation
Precipitation strengthening increases the strength, but destroys the continuity of the material, and causes distortion in the second phase and the surrounding matrix, increasing the transition temperature . However, in the microalloyed steel produced by controlled rolling and controlled cooling, microalloyed niobium , vanadium , titanium and other elements play a role in precipitation strengthening and also refine grains, which improves both strength and toughness .
3 Epilogue
By taking the following measures, the strength and toughness of the trial-produced EH36 steel plate meet the requirements of ship plate certification, and other toughness indicators, including strain aging impact , non-plastic transition temperature ( NDT ) test , and welding performance, all meet the requirements of classification society certification, and there are relatively Large margin .
( 1 ) Low-carbon niobium , vanadium and titanium are reasonably designed for micro-alloying composition to reduce the content of harmful elements in steel including P , S and residual elements As and Sn .
( 2 ) Improve the purity of billets and improve the low-magnification structure of billets .
( 3 ) The optimization of controlled rolling and controlled cooling process and normalizing process ensures that fine and uniform ferrite plus pearlite can be obtained .
references
- Yong Qilong . The second phase M in iron and steel materials ] . Beijing: Metallurgical Industry Press , 2009 : 25-26.
- Wang Youming, Li Manyun, Wei Guang . Controlled Rolling and Controlled Cooling of Steel M. Beijing: Metallurgical Industry Press , 2009.
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